For more than eighty years, traffic light signals and pedestrian signals have guided motorists and pedestrians at potentially dangerous roadway intersections to prevent vehicular accidents. The design and construction of these illuminated traffic signs have remained relatively unchanged until recent development of light emitting diode (LED) technologies.
With the development of brighter LEDs, these semiconductor devices are now being used as light sources in many lighting applications, including illuminated traffic signs, which have traditionally used conventional light sources, such as incandescent, halogen and fluorescent lamps. This is due in part to the fact that LEDs have many advantages over conventional light sources. These advantages include longer operating life, lower power consumption and smaller size.
However, since illuminated traffic signs must comply with a specified standard, e.g., National Transportation for ITS Protocol (NTCIP), current methods for building illuminated traffic signs with LEDs are complicated and require expertise in many areas, including video camera systems, video system viewing angle, and viewing distance analysis, as well as equipment setup and calibration. These conventional methods typically involve calibrating the illuminated traffic signs by generating a white base signal before setup in the field as a reference to ensure that the illuminated traffic signs are manufactured, installed and operational as specified in the required standard. This reference point will then be used in the field as a calibration point to produce white balanced signs with proper ratio of red, green and blue. In order to produce these white balanced signs, the right ratio of color binning is required, which involves a series of mix and match evaluations to generate a white base point. This process involves thousands of color mixing matrixes by varying the luminous intensity, dominant wavelength and the forward voltage.
A concern with these conventional illuminated traffic signs is that the light output of the LEDs degrades over time due to current stress, which causes the calibrated colors and brightness of the signs to deviate from their initial settings. These traffic signs may be recalibrated to compensate for LED light output degradation. However, recalibration requires a complete measurement tool setup and is very time consuming.
In view of this concern, there is a need for an illuminated traffic sign and method for providing illumination that can compensate for LED light output degradation over time.